Polycrystalline ZnSx,Se1–x thin films, deposited by co-evaporation of ZnS and ZnSe were characterized through spectral transmittance and AFM (Atomic Force Microscopy) measurements in order to determine the effect of thickness variation and surface roughness on their optical constants. A special method was implemented to determine the optical constants (absorption coefficient α, refractive index n) and thickness variation Δd of non-uniform ZnSxSe1–x, based upon spectral transmittance measurements and numerical simulation of the transmission spectrum performed with the help of an iterative computer algorithm based on the Powell's Convergence method. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)